In recent years, the number of diabetic patients has increased due to the westernized dietary habits and chronic lack of exercise and the like. In diabetic patients, the insulin secretion and insulin sensitivity are both reduced by chronic hyperglycemia and this invites further rise in blood sugar levels to deteriorate the condition of the disease. As the therapeutic agents for diabetes, biguanide drugs, sulfonylurea drugs, glycosidase inhibitor drugs, insulin resistance improving agents and the like have been used. However, as the side effects associated with these agents, lactic acidosis is reported for the biguanide drugs, and hypoglycemia is reported for sulfonylurea drugs and diarrhea is reported for the glycosidase inhibitors, and therefore the actual condition is that the development of therapeutic agents for diabetes having a new mechanism of action different from that of these drugs is earnestly desired.
It was reported (see Non-patent Document 1) that phlorizin, a natural-occurring glucose derivative, inhibits reabsorption of excess glucose in the kidney by inhibiting sodium-dependent glucose co-transporter 2 (SGLT2) present in the S1 site of renal proximal convoluted tubule and promotes glucose excretion to exhibit lowering of blood sugar levels, and since then the study of therapeutic agents for diabetes based on the SGLT2 inhibition has been eagerly conducted.
For example, in Japanese Patent Publication 2000-080041 A (Patent Document 1), International Publication Nos. WO01/068660 (Patent Document 2), WO04/007517 (Patent Document 3) and the like, compounds used as SGLT2 inhibitors are reported. However, since phlorizin and the compounds described in the above-described Patent Applications have a glucose moiety as a common partial structure, and it is regarded as a problem that on oral administration, these compounds are easily hydrolyzed by glucosidase present in the small intestine or the like, and their pharmacological action quickly disappears. In the case of phlorizin, it is reported that phloretin, the aglycon of the phlorizin, strongly inhibits a sugar transporter of the facilitated diffusion type. For example, when phlorizin is intravenously administered to a rat, an adverse effect of reducing the intracerebral glucose concentration is reported (refer to, for example, Nonpatent Document 2).
Then, in order to prevent such decomposition and improve absorption efficiency, some attempt to convert such compounds to prodrugs thereof has been carried out. When a prodrug is administered, it is desired that the prodrug be appropriately metabolized in or near a target organ to change into an active compound. However, since various metabolic enzymes are present in a living body and individual variability is considerable, in many cases, it is difficult to provide an action of a prodrug stably. Further, the conversion of the glycoside bond of the compound to a carbon-carbon bond has been attempted (refer to Patent Documents 4 to 8). However, further improvement of the properties as a drug, including activity, metabolic stability and the like are demanded.    Patent Document 1: Japanese Patent Publication 2000-080041 A    Patent Document 2: International Publication No. WO01/068660 Pamphlet    Patent Document 3: International Publication No. WO04/007517 Pamphlet    Patent Document 4: US Patent Publication 2001/041674 A    Patent Document 5: US Patent Publication 2002/137903 A    Patent Document 6: International Publication No. WO01/027128 Pamphlet    Patent Document 7: International Publication No. WO02/083066 Pamphlet    Patent Document 8: International Publication No. WO04/013118 Pamphlet    Non-patent Document 1: J. Clin. Invest., 93, 1037 (1994)    Non-patent Document 2: Stroke, 14, 388 (1983)